1. Field of the Invention
The present invention is directed to a ventilation system, and in particular to a ventilation system having either a fixed speed drive or a variable frequency drive, and an automatic torque converter intermediate the drive and the fan.
2. Prior Art
Many buildings, housings, cabinets and other enclosures require ventilation systems and have changing ventilation needs due to temperature changes. Ventilation systems come in a variety of configurations and are well known and widely used in kitchens and other areas that need to exhaust air and provide make-up air to replace the exhausted air. Many systems have been developed that control the ventilation rate by actuating fans and/or varying fan speed to respond to changes in temperature or pressure, often based upon electronic sensor input to a programmable logic unit (PLU) or other electronic device. Systems often use sensors and complicated controls varying motor speed to actuate and control motor speeds for varying the ventilation rates as the needs change.
A shortcoming with ventilation systems is due to the code requirements for designing the proper size fans, ducts, hoods and other ventilation equipment. The codes are based on criteria that may not actually match the physical processes of the area. Over-design of system components leads to excess exhaust air and also requires corresponding excess make-up air at some ventilation levels. Some systems may provide sufficient ventilation, but require greater energy to drive the system and over design for the actual needs. In addition, at higher operating speeds, such systems provide more torque than is required, placing a greater load on the motor, thereby shortening the life of the equipment.
It can be appreciated that a better understanding of the variability of thermal processes and their direct interrelationships with changes in pressure would lead to an optimized ventilation engineered design. The ideal gas law of physics should be used in determining ventilation rates. The ideal gas law is expressed by the equation pV=a constant for a fixed mass of gas. Therefore, pV=nRT, where p is the pressure, V is the volume of the gas such as air, n is the number of molecules (or moles) of the gas, R is the gas constant, and T is the absolute temperature, typically expressed in degrees Kelvin. It is appreciated that with this relationship, if the temperature increases, such as occurs through cooking or other heat generating activities, to maintain constant pressure, the number of molecules must be increased or the volume must be increased. Gas expands as it is heated in proportion to the temperature. Therefore, to maintain the same pressure, the ventilation rate must move a greater volume of air. However, the number of molecules being ventilated does not necessarily change. This can be explained by the expansion of the individual molecules as the temperature increases. Since the same number of molecules is ventilated, the mass occupying a greater volume moved remains constant; therefore the resistance of the gas and the force necessary to move them remains constant. To move a greater volume, fan speed is increased, while torque is kept constant. Conversely, a cooler gas is denser and provides greater resistance, so that if constant torque is provided, the fan will automatically slow down. With the prior art systems, an increase in motor speed also increases the torque, thereby overdriving the system.
It can be appreciated that a new and improved ventilation system and method for controlling ventilation fans is needed that overcomes the problems of the prior art. Such ventilation systems should provide for ventilation performance that matches the actual physical properties of the system. Such a system should provide fans driven at various speeds, but with a constant torque. The present invention addresses these, as well as other problems associated with ventilation systems and methods of controlling ventilation systems.
The present invention is directed to a ventilation method and system, and in particular to a ventilation system and ventilation control method based on the ideal gas law that utilizes constant torque to drive ventilating fans at varying speeds. The ideal gas law equation of state is expressed as:
pV=nRT
where p is the pressure, V is the volume of the gas, n is the number of moles or molecules of the gas, R is the gas constant, and T is the absolute temperature. As air behaves much like an ideal gas at room temperature, the ideal gas law can be applied to ventilation systems.
Using the absolute gas law as the basis for ventilating a cabinet, chamber, room or other area or enclosure, it can be appreciated that if substantially constant pressure is desired, as in many applications, the ventilation rate proportional to temperature. In other words, as the gas expands due to an increase in temperature, the volume of gas ventilated increases, but the actual number of gas molecules remains the same. As the gas expands, it becomes more buoyant and less dense, thereby exerting less resistance. Conversely, a fan driving the gas will have greater resistance at lower temperatures with a denser gas. Using this as the basis for driving the fan, if a torque converter is utilized, thereby maintaining constant torque on the fan, greater resistance results in a slower fan speed and thereby a lower ventilation speed. As the gas molecules are actually smaller, the same number of molecules is driven as at a higher temperature wherein the molecules are less dense and more buoyant. Higher temperatures increase gas volume, but also decrease resistance, so that the fan speed increases under constant driving torque.
Applying this to a real world ventilation system, such as a kitchen or a computer cabinet, the ventilation fan is driven by a motor with a transmission intermediate the fan and the motor. Once a base ventilation rate is determined for room temperature, the torque settings to drive the fan utilizing a motor of known size can be determined. With a torque converter applied, the torque to the fan is constant. Therefore, as the room temperature decreases, the speed of the fan slows, due to the less buoyant cooler, denser gas. When the temperature increases, the more buoyant lighter gas provides less resistance and the fan or fans speed up, moving a larger volume of air. In a preferred embodiment, torque converters are applied to the make-up air fans as well as exhaust air fans in a system to maintain a proper ventilation rate for the chamber. It can be appreciated that no controls are needed to turn on the system or actuate it in response to temperature changes as the torque converter automatically provides the proper driving force to each fan in response to temperature changes and the corresponding gas density and resistance changes due to the constant torque applied.
These features of novelty and various other advantages, which characterize the invention, are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described a preferred embodiment of the invention.